Add a basic implementation for MemoryRandomAccessFile (#136)

* Add a basic implementation for MemoryRandomAccessFile

Add a basic implementation for `MemoryRandomAccessFile` so that
`MemoryFile.open`/`openSync` can work.  Currently all functions
related to `lock`/`unlock` remain unimplemented.

* Make MemoryRandomAccessFile.readIntoSync and writeFromSync more efficient
7 files changed